feat(config): implement spec-compliant devcontainerId derivation

[skevetter]

Summary

Replaces the old devcontainerId derivation (SHA-256 hex prefix of workspace folder path, 20 chars) with the algorithm from the official devcontainer CLI source (devcontainers/cli at src/spec-common/variableSubstitution.ts):

1. Serialize ID labels as JSON with sorted keys (matching JSON.stringify(labels, Object.keys(labels).sort()))
2. SHA-256 hash the UTF-8 bytes of the JSON string
3. Interpret the 32-byte hash as a BigInt and convert to base-32 (digits 0-9, letters a-v — same as JavaScript's BigInt.toString(32))
4. Left-pad with '0' to exactly 52 characters

The new ComputeDevContainerID(labels map[string]string) function accepts arbitrary labels, supporting the --id-label CLI flag which overrides defaults. DeriveDevContainerID now takes both workspace folder and config file path to construct the spec-required default labels (devcontainer.local_folder and devcontainer.config_file).

LegacyDeriveDevContainerID is retained for backward compatibility. Container discovery is unaffected — it uses workspace UID via GetIDLabels, which is independent of the ${devcontainerId} variable substitution.

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Configuration used: Organization UI

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Run ID: f3074f1a-f280-4b54-b375-3067e72491d5

📥 Commits

Reviewing files that changed from the base of the PR and between 9563e32 and 105320a.

📒 Files selected for processing (3)

• pkg/devcontainer/config.go
• pkg/devcontainer/config/substitute.go
• pkg/devcontainer/config/substitute_test.go

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